I wrote a set of rules for playing with plastic Army Men, for my three sons, back in 1998. That was a segway into D&D, when they turned 13. Today, I still game with my two younger sons, with D&D; my middle son still plays the Army Men game rules with me, and his 30-something cousin, my nephew: the rules play fast, and they play decisively, but they're also a lot of fun.
For the Army Men game, the investment is minimal. My nephew decided to invest in his own figures and tanks. He spent around $40 USD to outfit himself with 200 men, and 12 Tanks. We're planning a huge city fight, using MDF buildings he made using his carpentry shop, along with his 50W Laser Cutter/Engraver system. Table size will be 12 feet by 30 feet, just like the last big game, in his shop… We have more fun than a human being ought to be allowed. ;-)
RPG's are the next cheapest gaming investment, after Army Men games, IMO. I've been investing in the RPG hobby since 1980. While I've sunken around $3,000 USD-$4,000 into it, that is pitiful when you divide the money by the number of years: $4,000 USD / 39 years = $102.56 USD per year. I spend that much seeing 10 movies in the theater -- without popcorn or a drink! Add those in, and I'm down to five movies in a year!
My Army Men games with my sons usually last for 6-18 hours, depending upon size. My RPG sessions with them, and a couple of friends, last six hours each. It is incredibly inexpensive entertainment.
With role playing games, my sons have made terrible mistakes, without being hurt by them. They have grown in wisdom, learning from their mistakes through their imaginary character's actions, what to do in various bad situations. Had they made those mistakes in a real-life situation, they may not have survived. If their character dies, they roll up a different one, and begin anew. Real-life does not have a reset button. With RPG's, there is no script, as found in PC and video games. There is no "one way" to win. It is always fluid, and it is always changing. They are always adjusting to the ebb and flow of the situation, just like in real-life. Cheers!